This invention relates to an improved arc-furnace and investment casting apparatus and more particularly to an apparatus ideally suited for casting reactive and relatively high melting point metals such as titanium.
Electric arc furnaces have been used for melting a variety of metals and alloys in order to prepare the material in the form of ingots or castings. Various advantages result from an arc melting process including the following:
1. Almost any metal can be quickly and easily melted including those that are chemically reactive and have very high melting points such as tungsten which has a melting temperature of about 3,400.degree. C.
2. The melting may be done in an enclosed chamber containing an inert gas such as argon or helium which prevents contamination of the metal by atmospheric gases such as oxygen or nitrogen.
3. The molten metal may be held in a relatively nonreactive container thereby avoiding contamination which can be introduced if one were to use a graphite, refractory oxide or heated metallic crucible.
4. Power supplies and accessory equipment are relatively inexpensive.
Arc melting uses a non-consumable tungsten cathode which emits electrons through an arc under the influence of an electric field in the direction of an anode. The anode normally contains the material to be melted. The concentration of electrons at the anode causes release of heat which is responsible for the generation of high temperatures. This is taught in Campbell and Sherwood, "High Temperature Materials and Technology", John Wiley & Sons, New York (1967).
Various modifications of an arc furnace have been suggested to permit manufacture of small castings, for example, by allowing molten metal to flow into a split copper mold, see Hepworth, M. T., Journal of Metals, volume 14, page 411 (1962) and Crow, J. E. and Sweedler, A. R. Review of Scientific Instruments, volume 44, page 1398 (1973). However, copper molds are relatively expensive and in many applications, such as for the manufacture of dental castings, would be uneconomical since a special mold is needed for each individual dental casting.
For this reason, the "lost wax" or "investment" technique is normally used for the manufacture of dental castings. In this process, metal is cast into a mold of gypsum or other ceramic material which is preheated to about 1500.degree. F. (815.degree. C.) at the time the metal is cast. Typically, the hot mold is heated in a furnace from which it must be removed for placement in a casting machine. This transfer must occur quickly, preferably in less than one minute, so that the mold temperature will not decrease significantly before the casting process begins.
Various patents suggest other melting and casting processes for the manufacture of dental castings. Thus, Emerick, in U.S. Pat. No. 4,150,707, teaches the use of an arc furnace which melts material placed in a metal crucible having a central bottom opening. An investment cavity is positioned beneath this crucible opening for receipt of the liquid metal upon melting thereof by the arc furnace. Emerick's relatively small metallic crucible is well-suited for melting gold alloys but would not be adequate for melting reactive metals having significantly higher melting temperatures such as titanium since such metals would be contaminated by reactions with the crucible.
Schultheiss, in U.S. Pat. No. 3,955,612, discloses a device for melting metals in a closed gas pressurizable crucible. The contents of the crucible are heated by an induction melting process. The crucible is arranged on top of a molding cell which is connected by a valve controlled tapping passage from the crucible to the cell. The melt is tapped under pressure into an evacuated mold. A center tap hole in the crucible is opened and closed by means of a central rod which projects down through the crucible and the molten metal and fits into the central passage. The crucible and central rod, while suitable for melting gold alloys, are not adequate for melting more reactive metals, such as titanium, which would attack both the rod and the crucible.
Ida et al in IADR Abstract No. 397 from the 1980 IADR meeting discloses a dental casting machine known by the tradename CASTMATIC which was used for the casting of titanium and titanium alloy castings. The CASTMATIC machine uses a graphite crucible which purportedly is non-reactive with the cast material. However, reactions between graphite crucibles and molten titanium have been described by A. D. and M. K. McQuillan "Titanium", Buttersworth Scientific Publications--London, pages 70-72 (1956). The substitution of cooled copper crucibles would eliminate problems of contamination, but the copper must be adequately cooled and Ida indicates no provision for such cooling in his design.
The present invention contemplates improved melting and investment casting of such reactive and refractory metals particularly for dental castings.